1. Field of the Invention
The invention relates to digital communications systems and, more particularly, to a receiver architecture that is characterized by a relatively low intermediate frequency (IF) section, followed by polyphase filter that results in enhanced image rejection, and a digital I/Q demodulator.
2. Description of the Related Art
Digital modulation and demodulation techniques incorporating I/Q (In-phase/Quadrature) modulators and demodulators are widely used in communication systems. I/Q demodulators are abundantly discussed in the technical literature. See, for example, Behzad Razavi, RF Microelectronics, Prentice Hall (1998) and John G. Proakis, Digital Communications, McGraw-Hill (1995). There exists also patent art related to the technology of I/Q modulation and demodulation: U.S. Pat. No. 5,974,306, entitled “Time-Share I/Q Mixer System With Distribution Switch Feeding In-Phase and Quadrature Polarity Inverters” to Hornak, et al; U.S. Pat. No. 5,469,126, entitled “I/Q Modulator and I/Q Demodulator” to Murtojarvi.
Examples of digital communications system applications that incorporate and standardize I/Q modulation and demodulation include the GSM (Global System for Mobile Communications), IS-136 (TDMA), IS-95 (CDMA), and EEEE 802.11 (wireless LAN). I/Q modulation and demodulation have also been proposed for use in Bluetooth wireless communication systems.
Bluetooth is a low-power radio technology being developed with a view to substituting a radio link for wire and cable that now connect electronic devices, such as personal computers, printers and a wide variety of handheld devices, including palm-top computers, and mobile telephones. The development of Bluetooth began in early 1998 and has been promoted by a number of telecommunications and computer industry leaders. The Bluetooth specification is intended to be open and royalty-free and is available to potential participants as a guide to the design and development of compatible products.
The Bluetooth system operates in the 2.4 GHz ISM (Industrial, Scientific, Medical) band, and devices equipped with Bluetooth technology are expected to be capable of exchanging data at speeds up to 720 Kbs at ranges up to 10 meters. This performance is achieved using a transmission power of 1 mw and the adoption of frequency hopping protocols to avoid interference. In the event that a Bluetooth-compatible receiving device detects a transmitting device within 10 meters, the receiving device will automatically modify its transmitting power to accommodate the range. The receiving device is also required to operate in a low-power mode as traffic volume abates, or ceases altogether.
Bluetooth devices are capable of interlinking to form piconets, each of which may have up to 256 traits, with one master and seven slaves active while others idle in standby nodes. Piconets can overlap, and slaves can be shared. In addition, a form of scatternet may be established with piconets overlapping, thereby allowing data to migrate across the networks.
The invention addressed herein is driven by the long-standing requirement, applicable with equal force to Bluetooth designs, to eliminate, or least minimize, the need for external filters commonly encountered in the design of contemporary double-conversion communications receivers. An example of the typical double-conversion receiver architecture is illustrated in FIG. 1. That architecture requires a first bandpass RF filter 21 disposed between antenna 10 and RF amplifier stage 20. A second bandpass RF filter couples the output of RF amplifier 20 to a first input of mixer 30. The primary function of RF bandpass filters 21 and 22 is to effect front-end selectivity, thereby enhancing the receiver's image response performance, as well as affording protection against spurious responses related to, for example, intermodulation and cross-modulation phenomena. However, because the selectivity provided by filters 21 and 22, in general, varies inversely with the insertion loss thereby caused, the level of selectivity attainable is limited by system design constraints. Furthermore, RF bandpass filters are not conveniently realizable in integrated circuit form. Consequently, the necessity of coupling outboard RF filters at strategic points to otherwise integrated receiver circuitry increases the manufacturing complexity and cost, as well as the physical size of the receiver.
With continued reference to the receiver architecture depicted in FIG. 1, the RF carrier is first converted to IF in mixer 30. The LO signal to mixer 30 is synthesized from a phase-locked oscillator that includes a VCO 50 and phase-locked loop (PLL) 40. The output of mixer 30 is coupled to an IF bandpass filter 31. The paramount functions of the IF bandpass filter are to establish channel selectivity and to define the noise bandwidth of the receiver. The output of IF bandpass filter 31 is coupled to the input of amplifier 60. The output of amplifier 60 is coupled to one input of demodulator 70. The-second input to modulator 70 is derived by processing the output of amplifier 60 through quad tank 61. The output of demodulator 70 is filtered by low-pass filter 71, and NRZ data is recovered in a bit slicer 80 that operates synchronously with the SCLK signal.
What is notable with respect to the above receiver architecture, and underscored in FIG. 1, is the necessary inclusion of no fewer than four outboard filters, BPFs 21, 22 and 31, and quad tank 61. These filter elements are not readily realizable with resort to contemporary integrated circuit technology. Bandpass RF filters 21 and 22 are frequently implemented with surface acoustic wave (SAW) devices, and the IF bandpass filter 31 often requires a crystal filter. Quad tank 61 may be predictably constructed from lumped passive circuit elements. It is readily appreciated that the necessary inclusion of these filter elements frustrate, or at least compromise, the objective of achieving a small, compact and easily transportable communications receiver. Accordingly, what is desired is a receiver architecture that satisfies system requirements such as selectivity, image rejection and noise figure, while limiting the dependence on non-integrable frequency-selective components.